The invention concerns a device for removing material screened or filtered out of a liquid flowing in a channel, consisting of an unpowered cylindrical grating that slants down, partly immersed in the liquid, to the bottom of the channel, with the bars of the grating extending over its total length leaving longitudinal interstices between them, and that merges into an upward-sloping section in the form of a screw-type conveyor that comprises a housing, a shaft, and a worm and conveys the screened-out material to a depositing point, whereby the worm picks up the screened-out material as it scrapes the bars of the grating and conveys the material upward, constantly keeping the grating clean. Although the device is especially intended for water-treatment plants, it can also be employed in a practical way in other fields of technology, in the textile and plastics industries, etc., for example. The device is especially practical when employed with narrow channels, 100 to 500 mm wide, and/or when the interstices between the bars must be narrow in order to pick up and remove fine material.
A device of this kind is known from German OS 2, 142 540. It consists essentially of a screw conveyor, housing shaft, and worm, with a motor mounted on top that transmits its rotation to the shaft and hence to the worm. The lower section of the conveyor housing, where it is immersed in the liquid, is completely occupied by the grating, which consists of bars that extend over its total length with longitudinal interstices between time, parallel to the axis of the shaft, and along its cylindrical surface. The grating bars are not braced transversely inside the grating and are essentially rectangular in cross-section, with the longer sides of the rectangle extending radially outward from the shaft. There are several drawbacks to bars, and to the interstices between them where the screened-out material precipitates, that are designed in this way. First, the bars are comparatively unstable, and it is difficult to position them precisely enough to ensure that the worm will come into contact with as many points as possible on the inner surface of the grating as it rotates against it. It is in fact only where such contact occurs that the deposited material will be entrained. Where there is no contact, the material can accumulate and clog up the grating, detrimentally increasing its hydraulic resistance. Furthermore, once the material has accumulated to a certain thickness, the rotating worm can eventually force it back out through the interstices into the filtered water, destroying the function of the grating at that point. This possibility increases with the physical and structural instability of the bars to the extent that they will resiliently bend out radially subject to the force of the worm, increasing the width of the interstices. When, as at this state of the art, the longer side of the rectangular cross-section of the bar extends radially outward, the moment of resistance to the outward bending that increases the width of the interstices is especially deleterious. Furthermore, since the open cross-section expands only in proportion to the radius and hence only slightly along the direction of penetration, radially in relation to the axis of the shaft, that is, when the bars are designed and arrayed in this way, material that penetrates the narrowest point of the grating can settle in the interstices, get wedged in, and hence reduce the free surface of the grating, also increasing the hydraulic resistance. When the material includes rigid objects like rocks, branches, etc., they can, due to the comparatively resilient structure of the bars, get caught between them outside of the narrowest section of the grating, permanently expanding its surface in the vicinity of the objects, to the detriment of reliable precipitation and allowing even larger objects to penetrate at these points. The inner ends of bars in the known device can also be in the form of knives that cut up materials with long fibers. This is another drawback in that the sensitive edges of the knives can be damaged due to the abrasiveness of their contact with the worm. The blades can in particular become bent by hard objects. The wear produced by the worm is accordingly also not negligible. It is particularly deterimental when long fibers are severed because the resulting shorter sections penetrate very readily through the interstices back into the cleaned liquid and do not settle at all.
A similar device, with a slanting shaft and conveyor worm, that is, is known from U.S. Pat. No. 2 929 504. The housing for the worm, however, extends all the way down to the bottom of the channel, and the grating replaces only part of its wall. Since the grating is actually in the form of a sheet of metal with a number of longitudinal slots, the interstices do not extend very far axially and their width is very stable. The risk of the interstices varying in width due to the displacement of the resilient bars is accordingly eliminated. Other drawbacks, however, are associated with a perforated sheet of this type. In particular, a considerable amount of stringing occurs during precipitation, meaning that fibrous material settles and forms strings at the edges of the perforations, preventing the liquid from penetrating. This devcice accordingly does not involve any continuous longitudinal interstices, and the ends of the slots create in conjunction with the rotating worm a shearing or severing mechanism that prevents the worm from conveying all of the precipitation material upward, whereas most of it gets trimmed off and penetrates into the cleaned portion of the liquid.